Faraway Planets Orbiting Distance Stars

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Smaller faraway planets...

Neptunians Beyond the Solar System

Neptunian exoplanet.
NASA artist imagines the newly discovered Neptune-sized extrasolar planet circling the red dwarf star Gliese 436. Here, the planet appears gaseous like Jupiter, with a cloudy atmosphere. However, astronomers don't know yet if the newly discovered smaller planets are gaseous like Jupiter, or rocky like Earth and Mars.  click to enlrage
Astronomers looking for faraway planets orbiting distant stars have found a new class of such objects beyond our Solar System.

Far smaller than exoplanets found earlier, these planets are about 10 to 20 times the size of Earth.

That makes them about as big as the known planet Neptune and, therefore, they are referred to as neptunian extrasolar planets.

Exoplanets discovered earlier are the size of the known gas-giant planet Jupiter. The Neptune-sized planets prove that planets the size of Jupiter aren't the only ones out there, according to the astronomers.

Other systems. Neptune, Jupiter and Earth are in our Solar System. The new small planets are in solar systems around the stars Gliese 436, 55 Cancri, and mu Arae.

One of the newly-discovered planets is near three other previously discovered exoplanets orbiting the star 55 Cancri. Together, they form the first-ever-known four-planet system beyond our own system.

The several discoveries were made by astronomers at the Carnegie Institute of Washington, University of California at Berkeley, University of Texas at Austin, University of Lisbon in Portugal, Marseille Astrophysics Laboratory in France, and University of Geneva in Switzerland.

Just How Massive Is That Planet?
Astronomers compare the planets in our Solar System with Earth. Rather than list a planet's mass in kilograms, they compare its mass to Earth's mass. For instance, let's say Earth's mass is 1 and the mass of Jupiter is 318, then Jupiter is 318 times more massive than Earth.
Planet Mercury Venus Earth Mars Jupiter Saturn Uranus Neptune Pluto
Mass 0.055 0.88 1 0.107 317.8 95.2 14.48 17.2 0.005

Are they rocky? To date, nearly 140 extrasolar planets have been discovered. All of the earlier discoveries have been larger planets of the size of Jupiter.

Astronomers wonder if the newly discovered smaller planets are spheres of gas like Jupiter or hard look like Neptune with a heart of rock and ice surrounded by an atmosphere of hydrogen and helium.

Because they are smaller than Jupiter, it is possible they are made of rock and ice, rather than gas. Like Earth, they may have formed through gradual accumulation of rocky bodies. A planet the size of Neptune may not have enough mass to hold onto a lot of gas.

These planets stay close to their parent stars, whipping around them in a matter of days.
  • One of the planets circles the small, cool, reddish dwarf star Gliese 436 about every 2.5 days at just a small fraction of the distance between Earth and the Sun – about 2.6 million miles. Earth is some 98 million miles from the Sun.

    The planet at Gliese 436 is only the second planet found orbiting a so-called M dwarf, a type of low-mass star about 40 percent the size of our own Sun. Gliese 436 is located in our galactic backyard, just 30 lightyears away in the constellation Leo. The fact that red dwarfs are the most abundant type of star in our Milky Way galaxy suggests to astronomers there may lots more neptunian planets.

  • Another of the planets races around 55 Cancri, a yellow star like our own, in just under three days. Its distance from its star also is just at a fraction of the distance between Earth and the sun, at approximately 3.5 million miles. Three larger planets also revolve around 55 Cancri every 15, 44 and 4,520 days.

    The outermost of the 55 Cancri planets, discovered in 2002, is still the only known Jupiter-like gas giant to reside as far away from its star as our own Jupiter is from the Sun. The star 55 Cancri is about 5 billion years old and a a bit lighter than our Sun. It 41 lightyears away from us in the constellation Cancer.

  • The planet discovered by European astronomers orbits the bright star mu Arae, which is seen in Earth's nighttime sky in the southern constellation Ara (the Altar). It completes a full revolution around the star in 9.5 days and is the second planet discovered around this star. A Jupiter-sized planet already had been found orbiting Mu Arae on a 650-day period. Astronomers have found hints of yet another another companion near mu Arae, which is about 50 lightyears away from Earth and bright enough to be seen with the unaided eye.

    With a mass of only 14 times the mass of the Earth, the new planet lies at the threshold of the largest possible rocky planets, making it a possible super-Earth object. Uranus, the smallest of the giant planets of our Solar System has a similar mass. However Uranus and the new exoplanet differ so much by their distance from the host star that their formation and structure are likely to be very different.
How were they found? The discoveries were made using a technique called radial velocity, in which a planet's gravitational pull is detected by the wobble it causes in the parent star.

A star's movement, including any wobbling is calculated by measuring the Doppler effect on light arriving at Earth from the star. The wavelength of the star's light changes as it moves. That tells astronomers a lot about the star and objects near it, including mass, orbit and speed.
  • The planet at Gliese 436 was discovered during careful observation of 950 nearby stars with the W.M. Keck Observatory at Mauna Kea, Hawaii, and the Lick Observatory in California. The astronomers were able to spot such a relatively small planet, because the star it tugs on is small and more susceptible to wobbling.

  • The planet at 55 Cancri was discovered after 100 observations with the Hobby-Eberly Telescope at McDonald Observatory in West Texas. Archived data from the Hubble Space Telescope also was used.

  • The planet at mu Arae was discovered with the Harps spectrograph on the European Southern Observatory's 3.6m telescope at La Silla, Chile.
Future searches. While astronomers will continue searching for extrasolar planets with the telescopes already in place, NASA is planning to launch even more specialized equipment seeking more Earth-like planets.

Three planet-hunting projects in the works are named Kepler, the Space Interferometry Mission (SIM), and the Terrestrial Planet Finder (TPF).
  • The Kepler telescope is to be launched in 2007.
  • The SIM interferometer will be launched in in 2009.
  • TPF has two parts. The coronagraph is to be launched around 2014 and the interferometer is to be launched around 2020.

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